July 20, 2012 4:20 pm

They’re not the sleekest or the swiftest vessels ever launched from a Tacoma drydock, but when it come to sheer utility, the six concrete pontoons due to take to the water next week excel.

As big as small, windowless office buildings, the six concrete pontoons are the first of 44 due to be built in a leased Concrete Technology graving dock on Tacoma’s Blair Waterway over the next two years.

The six pontoons will be launched next week.

They’re vital parts of the state’s new state Route 520 bridge over Lake Washington in Seattle. The new bridge is due to replace the aging exiting 520 floating bridge in 2015.

The 44 Tacoma-built pontoons will provide extra buoyancy and stability for the chain of larger pontoons supporting the new bridge. Those pontoons are being built in Aberdeen.

Dave Becher, Washington Department of Transportation SR520 bridge construction manager, said the pontoons have been constructed over the last six months by a crew that averages 100 workers in Tacoma.

Kiewit/General/Manson is building all of the pontoons under a $586.6-million contract.

Each of the gray, rectangular Tacoma pontoons is roughly 100 feet long, 50 to 60-feet wide and 28 feet tall. The pontoons will be bolted on each side of the larger pontoons being built in Aberdeen once the pontoons are towed to Lake Washington from their construction sites.

The new bridge with six traffic lanes and a bicycle path is considerable wider than the existing four-lane 520 bridge, so the pontoons supporting the roadways had to be larger than the existing pontoons, said Becher.

But wider pontoons would not have fit through the Ballard Locks connecting the lake with Puget Sound, thus the need for bolt-on additions to the roadway pontoons to carry the bridge’s weight.

The pontoons range from about a foot-thick concrete at the bottom to eight-inches at the top. Inside, the boxy floating pontoons are divided into multiple watertight compartments to prevent a single leak from flooding the whole structure.

Reg Carson, the contractor’s Tacoma construction manager, said the contractor’s plan calls for the gate holding back the water from the dock on the Blair Waterway’s east side just south of East 11th Street to be opened Sunday with water flooding the long cavity.

As high tide approaches at about 6:30 p.m. Monday, a t-shaped pontoon will be positioned between two of the side-by-side pontoons and attached to them. As the tide rises, the additional pontoon will provide enough extra buoyancy to lift the first two pontoons off the dock’s bottom.

Tugs will then pull the pontoons out of the flooded dock and tie them up farther down the Blair for further outfitting. The same process will be repeated Tuesday and Wednesday until all six of the initial pontoons are removed.

The dock’s gate will be floated back into place and secured. Any remaining water will be pumped out. The contractor then will begin construction of the next set of pontoons, this time eight at once.

The first six pontoons will be moored to a pier on the Blair Waterway’s east side until the first Aberdeen pontoons are launched about Aug. 1. Both the Aberdeen and the Tacoma pontoons will be towed to Lake Washington where they will be bolted together, Becher said.

Once they’re secured, the roadway will be built atop them. The new bridge will be built slightly to the north of the existing structure and connected to the highway with new approaches. Once the new bridge is complete, the traffic will be diverted to the new bridge, and the old one will be dismantled.

Unlike the existing bridge, the new bridge won’t have a mid-bridge drawspan to interrupt traffic when large boats need to cross. The new bridge instead will have an elevated section 70 feet over the water to allow boats to pass under the structure unimpeded.

The Tacoma dock’s use to build the 520 pontoons isn’t the first time floating bridge pontoons were built in the Port of Tacoma. From 2006 through 2010, new pontoons for the Hood Canal Floating Bridge were constructed in the dock.

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